1. Field of the Invention
The present invention relates to a photosensitive member which has an amorphous silicon: germanium layer.
2. Description of the Related Art
Amorphous silicon: germanium (hereinafter referred to as a-Si:Ge), whose band gap is smaller than that of amorphous silicon (hereinafter referred to as a-Si), exhibits a high absorption characteristic toward long wavelength light. This enables many carriers to be generated to improve a sensitivity toward long wavelength light, so that a-Si:Ge is expected to be utilized as a photosensitive member for a printer with a semiconductor laser. Moreover, a high sensitivity in short wavelength region is retained to permit the application of the a-Si:Ge to plain paper copiers (hereinafter referred to as PPC) by regulating the emission spectrum of exposure lamps. In addition, the excellent characteristics of high absorption toward long wavelength light in the a-Si:Ge layer greatly improves disturbance of images caused by the interference of light which is frequently observed in conventional a-Si photosensitive members.
With these inherent features of the a-Si:Ge, various photosensitive members have been proposed which include the a-Si:Ge layer.
For example, Japanese Laid-Open Patent Application Nos. SHO 56-150753 and SHO 58-171039 and U.S. Pat. No. 4,491,626 propose to form an a-Si:Ge layer on a conductive substrate of a photosensitive member. However, these photosensitive members provide a problem that a quantity of carriers excited by short wavelength light in the vicinity of the surface, become substantially equal to that of carriers excited by long wavelength light in the vicinity of the substrate of the photosensitive member. As a result, both carriers move respectively to cross one another from the surface side to the substrate side and from the substrate side to the surface side in the a-Si:Ge layer of the photosensitive member. As both of these carriers have opposite polarities, polarity adjustment of the a-Si: Ge layer can be conducted by adding an impurity element, i.e. to adjust the transportability of holes and electrons. However, satisfactory result can not be obtained as it is generally difficult to ensure movability of both holes and electrons. Consequently, favorable transportability of carriers and suitable sensitivity are not attained. Moreover, a large thickness of the a-Si:Ge layer and a high Ge content to inhibit the interference of light hinder carriers excited by long wavelength light to move out of the a-Si:Ge layer as carriers are likely to be trapped in said layer, thereby causing reduction in sensitivity as well as in the charging capability due to many generation of excited carriers by the a-Si: Ge.
Further, other proposals have also been made to form the a-Si:Ge layer at the surface side of the photosensitive member in Japanese Patent No. SHO 56-150753 and U.S. Pat. No. 4,491,626. In these cases, a large thickness of the a-Si:Ge layer and a high Ge content in the layer to inhibit the interference of light hinder carriers excited by short wavelength light to move out of the a-Si:Ge layer thereby causing reduction in sensitivity as well as in the charging capability.
Also, U.S. Pat. No. 4,451,546 shows a photosensitive member having an a-Si layer, an a-Si:Ge layer formed on the a-Si layer and an a-Si layer formed on the a-Si:Ge layer. This photosensitive member connot absorb long wavelength light at a substrate side to a sufficient degree to thereby cause the interference of light.
As described above, a smaller band gap of a-Si:Ge compared with that of a-Si increases the absorbability toward long wavelength light to generate many carriers and improve sensitivity toward long wavelength light.
On the other hand, mere Ge doping makes a photosensitive member unworkable. For example, randomly high Ge doping increases a level of impurity elements in the band gap to thereby cause reduction of chargeability which should be said to the essence of photosensitive members. As a result, suitable electrostatic latent images cannot be obtained.
Moreover, a-Si:Ge has features of generating many carriers but conversely interfering the movement of the generated carriers. Accordingly, Ge content of meaninglessly high concentration as well as a large thickness of the a-Si:Ge layer hinder carriers to move to cause reduction in sensitivity and increase of residual potential. In addition, erasing of residual potential can not also be performed sufficiently to bring out unfavorable results for electrophotography such as generation of memory and so forth.
On the other hand, sufficient absorbability toward long wavelength light is required to inhibit the generation of the interference phenomena in electrophotography using coherent light as a light source such as in laser beam printers.